As long as your devotion to blue and black is less than seven, Phenax isn't a creature.
Creatures you control have ": Target player mills X cards, where X is this creature's toughness."
Phenax turns your whole board into a mill engine: tap creatures to grind opponents' libraries based on each creature's toughness. You ramp into fatties or token swarms with high toughness, then mill one or more players out before they can win. Early turns set up high-toughness defenders and recursion; mid-game you tap out to dump 30-100+ cards per opponent.
Mill scales explosively with high-toughness creatures and wide boards
Indestructible commander that's hard to remove and cheap to recur
Doesn't need combat damage, so walls and defensive creatures become threats
Blue/black gives premier card draw, removal, and counterspells to protect the plan
Mill is one of the slower win conditions and folds to fast combo/aggro
Adds passive incremental mill that punishes any untapping and stacks with your activations.
Add untap engines (Intruder Alarm, Kiora's Follower, Thousand-Year Elixir) and toughness payoffs to break parity and mill faster, then bolster protection with counterspells and graveyard-hate answers like Bojuka Bog to stop recursion. Lean into a token or 'enters' subtheme so Intruder Alarm goes infinite, and include strong card advantage (Rhystic Study, Mystic Remora) plus interaction so the slow clock survives the table.
Legendary Enchantment Creature — God
: Target player mills X cards, where X is this creature's toughness."